This invention relates to instruments for the measurement of intraocular pressures, known as tonometers, and in particular to applanation tonometers.
In an applanation tonometer the eye is observed through an applanating member which is applied against the eye with sufficient pressure to flatten a predetermined are of the surface of the cornea, so that the applied pressure is able to provide a measure of the intraocular pressure.
A known applanation tonometer is described in GB 862920, the contents of which are incorporated herein by reference. The applanating member comprises a doubling prism which produces a split image of a circular area of the eye against which the member is applied, the two semi-circular part images being relatively displaced as the pressure of application changes. A convenient datum position indicating that a specific area has been flattened is obtained when the two semi-circles are seen as a continuous S-line, as is explained in GB 862920.
The doubling prism is part of a sealed unit that forms the applanating member, the prism being sealed into the forward end of a tubular carrier, the rear end of which is sealed by a plane transparent plate. Because the applanating member is the only part of the tonometer that comes into direct contact with the patient, producing the member as a sealed unit that is detachable from the instrument make periodic sterilisation more convenient. More recently, however, stricter hygiene standards have created problems in this respect. In particular, it is significant that both the herpes simplex virus and the HIV virus have been found in tear fluids. Since the patient's tear fluid may be left on the applanating member there could be a risk of cross-infection if the member is not fully sterilised after use on each patient. To stock sufficient applanating members to allow for the time they are out of use creates a significant burden of cost, in addition to the cost of the sterilisation process itself. It would therefore be desirable to be able to limit the costs of meeting these stricter standards.